Swiss authorities said it was likely they had shot at another vehicle while fleeing along Flughafenstrasse, which becomes Route Douaniere once it crosses the border into France.

"How the offenders managed to leave Flughafenstrasse, which is in French territory, is a matter of investigations, which are being undertaken together with French authorities," they said. "The hunt for the perpetrators has so far been unsuccessful."

The general manager of the Grand Casino, Michael Favrod, said it was the first such hold-up on the casino close to the French-Swiss border and three minutes from Basel airport.

"The employees reacted very well," he said. "Such things (robberies) happen in casinos, so the employees have been trained to deal with such a situation. It was probably due to the training that no one was seriously injured."

Clients and employees have been offered psychological help.

The casino remained open on Sunday, said Favrod, noting clients were turning up "as usual. But we will have to see tomorrow, when the public learn about the incident from media reports".

The raid comes just two weeks after another heist at a casino in Geneva.

Police said on March 19 they had arrested a 20-year-old Frenchman living in Geneva who had robbed the casino with an assault rifle.

The man had fled in his car, which was found in the city. Police captured the man after identifying the vehicle.